What does a factory-second plug look like?
Exactly like a regular plug, which is why they're such a problem.
The ones with obvious defects get tossed. No major manufacturer wants
junk being sold with their brand on it. Then there are the others.
Most manufacturers are constantly updating and upgrading their production
machinery. The newest machines can hold the tightest tolerances. These
machines also have the highest priced "production time" because they're
still being paid for. By the time a machine is paid for it isn't holding
the dimensional tolerances it did when new, but is still producing a
usable product at a lower price than the new machine. Eventually it
wears to the point that the scrap count wipes out the savings and it gets
sold to a third world company making "odd brand" plugs.
The plugs on sale in a "big box" are being sold on a razor thin margin.
Even if it's a name brand, it's the least expensive production that
they're still willing to put their name on. While you may not be able to
tell a difference with the naked eyeball, if you sit down with a thread
micrometer and a pitch gage you'll spot it in a few minutes. You'll find
threads that are over/under sized, and not concentric with the plugs
axis. Sometimes you actually *can* see a problem. Hold a straight edge
against the threads so it lines up with the side electrode. The gap
between the straight edge and the electrode should be greater than the
depth of the threads. I've seen side electrodes welded on so far out
there was no way you could screw the plug into the hole without damaging
threads in the head.
Later,
Joe
